The ever increasing problem of toxic landfills and groundwater pollution has necessitated the development of a variety of sampling techniques for locating and tracking the migration of contaminants within underground formations. Conventionally, soil pore water samples are obtained either by the actual extraction of a natural soil sample or through the use of in situ samplers. In the former method, a soil core is collected and pore water is withdrawn from the sample by displacement, compaction, centrifugation, molecular absorption or suction. Conversely, in situ techniques rely primarily upon insertion of a collection vessel within the formation and creation of a vacuum to induce soil pore water flow into a collection vessel. Sampling devices developed for this purpose include vacuum pressure lysimeters, vacuum plates and tubes, membrane filter samplers and absorbent devices.
Generally speaking, most samplers designed for sampling liquids from unsaturated pores may be used to sample from saturated pores as well. However, samplers which are designed for sampling from saturated pores cannot always be used in unsaturated conditions. This is because the liquid in the unsaturated pores is held at less than atmospheric pressure. Further, when sampling in the vadose zone, it is desirable to avoid the actual extraction of the soil sample to the surface. Thus, vacuum or suction samplers commonly known as lysimeters are preferred sampling devices.
Prior art lysimeter probes generally comprise a basic tubular body the end of which has affixed thereto a suction cup, made of porous material, sintered metal or the like which is turned toward the ground. The suction cup itself forms the tip of a lysimeter probe and is mostly cylindrical or conical in shape. The suction cup is fragile as is the basic body which is comprised mostly of plastics or stainless steel. Consequently, the suction cup is sensitive to pressure and is often in danger of being damaged when inserted into the ground. To reduce the likelihood of damage, it has been known to pilot-drill a suitable hole with an auger followed by careful insertion of the lysimeter probe into the pilot-drilled hole. However such drilling is expensive, leads to the possibility of contamination on the surface and causes an undue disturbance to the formation thereby contributing to inaccurate results.
Another problem associated with prior art lysimeter devices is that the soil around the porous section is often so dry that samples cannot be effectively collected. The primary limiting factor under such conditions is the pore size of the lysimeter as well as the hydraulic properties of the soil i.e. the extent of unsaturation. It is therefore extremely important that the filter be in very close contact with the surrounding soil. Lysimeters employing pre-drilled holes do not achieve such close contact and therefore require application of a higher vacuum.
In view of the above, lysimeters which do not require pre-drilled holes have been proposed. These devices may be forcibly driven into the ground with a hydraulic ram or other means. However the forces generated while pounding such devices into the ground often cause damage to the lysimeter. This is especially true of the relatively fragile filter portions that are directly interconnected by threads or other means to the lysimeter body. As a result, hydraulically installed lysimeters have been proposed which provide a solid, sectional rod having fluid passages therethrough. In addition, lysimeters have been proposed which employ balloons to seal off a section of the tube or column so that a vacuum may be applied causing extraction of a fluid sample into the device. For example, U.S. Pat. No. 2,210,546 (Hassler) provides sealing balloons attached to the exterior of the device above and below the filter. Since the filter is not in intimate contact with the soil, sample extraction is not achievable in the vadose zone. Further, balloons create a poor seal against soil.
Still other devices such as U.S. Pat. No. 4,538,683 (Chulick) provide an exterior tube or casing having a screen disposed therein. This device cannot extract fluid samples from unsaturated soils nor is the casing adapted to be hydraulically installed. Fluid extraction requires that multiple concentric tubes be provided and aligned to position the various conduits. The device is both complex in structure and inefficient in operation.
In addition, many conventional lysimeter devices do not have the capability of withdrawing samples from different zones or intervals of the formation being sampled. Prior art sampling is often restricted to a single region location within the soil column. If a sample is desired from a different depth, a separate pilot-drilled hole must be made and the lysimeter must be either lengthened or shortened followed by reinsertion into the new hole. This approach is both time consuming and expensive.
Prior art devices that employ suction to extract the sample often require that a high vacuum be applied because the filter is not intimately in contact with the surrounding soil. Also, the applied vacuum in these prior art devices is not evenly distributed along the entire surface of the filter and therefore results in clogging. Lastly, prior art lysimeters create risks to both personnel and the environment since the drilling process may bring contaminated material to surface.